This invention relates to a method of producing carbonaceous blocks in a tunnel type furnace, and in particular, to a method of producing electrodes for the process of electrolytic reduction of aluminum in the molten state according to the Hall-Herault method.
The carbonaceous blocks are generally produced by pressure molding or extruding a carbon-carrying material mixture, such as petroleum coke, anthracite, gas black, graphite, and the like, with a binder material, such as tars and/or pitches. By calcining such blocks, the binder is baked to achieve desired mechanical and electric characteristics.
In general, anodes for electrolytically reducing aluminum are produced in so-called loop furnaces. For calcining such blocks, tunnel furnaces have also been proposed, wherein the blocks are coated with powder carbon to protect them against oxidation and stored into muffles mounted onto carriages which are pushed through the furnace in accordance with a preset thermal schedule. The temperature schedule, that is the heating rate, is a determining parameter in the method for producing high quality calcined electrodes.
During heat application, and in the temperature range from 200.degree. C. to 600.degree. C., and to a larger extent in the 400.degree. C. to 500.degree. C. range, the blocks being calcined release volatile organic substances originating from the decomposition and/or distillation of binder fractions in either gas or vapor form. Such volatile substances, which comprise essentially aromatic hydrocarbons, represent a big disadvantage especially from the standpoint of pollution and safety of the working areas, and accordingly, their effective disposal constitutes a serious problem to be solved by the methods of preparing carbonaceous blocks.
Disposal of volatile substances has not been effectively controlled heretofore, with the result that, particularly for the production of anodes for aluminum electrolysis, loop-type furnaces are almost exclusively used in practice, in spite of numerous methods of removing volatile decomposition products having been proposed.
Known from German Pat. No. 1508515 is a method of baking carbonaceous blocks in a tunnel type furnace, whereby tapping gases are supplied to the furnace inlet up to the level of the tunnel where binder vapor emission takes place, which gases are caused to flow in the same direction as the material being baked, extracted at that location along with the vapors released from the binder decomposition, and burned onto a catalyst, thereafter the burned, oxygen-free hot gases are redirected into the furnace.
While that method enables the use of a fumes scrubber to be avoided, which is highly important from the economical and environment control standpoints, it nevertheless has the disadvantage of providing for the extraction of the vapors and gases from the tunnel furnace system, which involves technical difficulties and is uneconomical.